A pressure-sensitive copying paper is generally produced by coating a coating composition mainly composed of microcapsules containing an oil having dissolved therein a substantially colorless electron donating dye (hereinafter referred to simply as microcapsules) and a coating composition mainly composed of an electron accepting developer (hereinafter referred to simply as a developer) on separate base papers or on the same base paper.
A pressure-sensitive copying paper is generally composed of an upper paper (CB (coated back) sheet) having a coated layer of microcapsules and a lower paper (CF (coated front) sheet) having a coated layer of a developer, or is composed of the upper paper, the lower paper, and an intermediate paper (CFB (coated front and back) sheet) having a coated layer of microcapsules on one surface thereof and a coated layer of a developer on the other surface thereof. In one embodiment, the upper paper is superposed on the lower paper so that the coated layers face to each other, or in other embodiment, one or more intermediate papers are inserted between the upper paper and the lower paper, and when pressure is applied to the assembly by a type-writer or hand writing, the microcapsules are raptured to release the oil containing the substantially colorless dye, which is then transferred onto the surface of the developer layer to form colored images.
The microcapsule-coated paper is usually prepared by coating a coating composition composed of microcapsules, a binder, a smudge preventing particles (stilts), etc., on a base paper. However, since a base paper for such a microcapsule-coated paper is porous and does not having sufficient water repellency, the components constituting the coating composition tend to permeate into the base paper after coating the coating composition.
In particular, if microcapsules in the coating composition permeate into the base paper, the rupture of the microcapsules by recording pressure and the transfer of the oil containing a substantially colorless dye onto a developer-coated paper are not effectively performed, thus reducing the coloring density.
Accordingly, in the case of coating on a base paper, it is required to coat a large amount of a coating composition as compared with the case of coating the coating composition on an impermeable support such as a polymer film.
As a method for disposing microcapsules on the surface of a base paper by preventing the permeation of microcapsules into the base paper, there are known the following techniques.
Japanese Patent Publication No. 21,499/68 describes a method wherein one of two kinds of water-soluble polymers forming a coacervate is added to a paper stock for making a base paper and a coating composition containing the other of the water-soluble polymers and microcapsules is coated on the wet paper on a wire cloth of a paper manufacturing machine. However, the above method is not said to be a practical method since it is difficult to coat the coating composition on the wet paper, and even if a coacervate is formed on the wet paper in the presence of a large amount of water, microcapsules permeate into the paper upon drying after the formation of microcapsules, whereby the effective prevention of the permeation of microcapsules cannot be performed.
U.S. Pat. No. 3,914,470 and British Patent No. 1,370,081 describe a method wherein a subbing layer composed of a re-wettable binder such as dextrin, polyvinyl alcohol, etc., and smudge preventing particles (stilts) is formed on a base paper and microcapsules only are coated thereon. According to the method, microcapsules exist on a binder layer and hence the rupture of microcapsules by recording pressure is effectively performed and the transfer of oil onto a developer layer is not disturbed by the binder layer, to give high coloring property. However, for sufficiently preventing the permeation of microcapsules in a base paper, it is required to coat a large amount of a high-viscous binder, but such a coating is impossible by an on-machine light-weight coating such as a size press of a paper machine, and thus it is required to coat the subbing layer and microcapsules by means of separate coating machines; which inevitably results in making the production step complicated and causing increased costs in making the product. Also, if subbing of the binder is sufficiently performed to prevent the permeation of microcapsules, the air resistance of the microcapsule-coated paper becomes very high, and hence there is no surface void for escape of air through the paper, which results in reducing the high-speed runnability of the sheets in a winder, a printing machine, etc. Thus, such a method is also lacking in practicability.
U.S. Pat. No. 3,565,666 and British Patent No. 1,222,187 describe a method for preventing the occurrence of rubbing smudge by forming a subbing layer of a latex. According to the method, the subbing layer of the latex acts as a cushion layer for microcapsules to prevent the occurrence of rubbing smudge. In this case, the subbing layer also acts to prevent the permeation of microcapsules into the base paper, thus improving the coloring property.
However, in the prevention of the permeation of microcapsules by forming a subbing layer of a binder such as the water-soluble polymer as described in aforesaid U.S. Pat. No. 3,914,470 and British Patent No. 1,370,081 and the latex as described in U.S. Pat. No. 3,565,666 and British Patent No. 1,222,187, the permeation of microcapsules is prevented by filling the voids of a base paper by the binder. Hence not only the permeation of microcapsules but also the permeation of a binder in the capsule coating composition are prevented, whereby the binder remains on the surface of the subbing layer. Thus, such methods are not always effective for the improvement of coloring property, and additionally the air resistance of the microcapsule-coated paper is increased according to these methods. Therefore, these methods are not totally suitable for practical use.
U.S. Patent No. 4,219,220 and British Patent No. 2,022,646 describe a method wherein inorganic solid fine particles and an adhesive are coated on a subbing layer on a base paper and a microcapsule-containing coating composition is coated on the subbing layer for preventing the permeation of the coating composition to improve the coloring property. However, in this method, the permeation of microcapsules is prevented also by filling the voids of a base paper and hence there are the same demerits that the binder disturbing the transfer of oil remains on the surface of the subbing layer and the air resistance of the microcapsule-coated paper is increased as the case of aforesaid U.S. Pat. No. 3,565,666 and British Patent No. 1,222,187.
Japanese Patent Application (OPI) No. 211,699/84 provides a method wherein boric acid and/or a borate is coated on a base paper as a subbing layer and a coating composition containing polyvinyl alcohol as an adhesive is coated on the subbing layer, whereby polyvinyl alcohol is aggregated by the action of boric acid or the borate to prevent the permeation of polyvinyl alcohol (The term "OPI" as used herein refers to a "published unexamined Japanese patent application"). However, when this method is applied for the coating of microcapsule-containing coating composition and a microcapsule-containing coating composition containing polyvinyl alcohol is coated on the subbing layer of boric acid and/or a borate, only polyvinyl alcohol as an adhesive remains on the surface thereof, whereby the transfer of the oil released from the microcapsules in the case of applying recording pressure is disturbed to reduce the coloring property.
As described above, a method of effectively performing the prevention of the permeation of microcapsules into a base paper to improve the coloring property while also preventing an increase of the air resistance has not yet been developed.